The present invention relates to a method of making fiber reinforced articles; and more particularly to a method of producing articles from glass fiber reinforced polymers or plastics which materials are commonly called FRP.
At this writing, the art of producing glass fiber reinforced plastic articles has progressed to the point where fiber prominence in the finished molded parts and depressions called "sink" over rib areas in the molded parts have been generally solved. With these problems solved it is now possible to make large thin contoured sections from FRP which were formerly made by bending sheet metal into the desired shape as is done for example in the forming of automotive hoods and fenders, and shower stalls, etc.. In order that the production of such articles could become commercially feasible, it was necessary to produce a moldable mixture of the glass fibers and the plastic prepolymers in a nonsticky, handleable condition; and this aspect of the art has been accomplished by the development of sheet molding compounds, commonly called SMC.
Sheet molding compounds, as the name implies, are sheets of mixtures of glass fibers and prepolymers that have been cured to a "b-stage" in which the prepolymers are no longer runny and in which the mixture is sufficiently solid that it can be handled as a sheet and to allow the glass and resin to flow together uniformly in the molding operation. Such sheets are fed to presses that flow, contour and cure the sheets in a process that is analogous in some respects to the stamping operation used by the sheet steel industry. Sheet molding compounds are made between separation sheets by putting a thin layer of a liquid hardenable resin on a lower separation sheet, a layer of chopped fibers on top of the resin layer, followed by another layer of the resin on top of the layer of chopped fibers. In the process, air is trapped in the layer of fibers between the two layers of resin, and a kneading action is utilized to remove this air from between the fibers. This is usually accomplished by passing the layers of materials through one or more nips of compression rolls. The problem of air removal is a difficult one, and at this writing the problem of air removal has limited the production of sheet molding compounds to a maximum thickness of approximately 1/4 inch. Still other problems exist with sheet molding compounds in that when the materials are deeply drawn or extruded, the fibers of the mixture stay behind and only its resin flows into the deeply drawn or extruded areas. Sheet molding compounds have still another limitation in that the fiber loadings (fiber-binder volume ratios) which the art has been able to produce are generally below 50%, with the commercial products being in the range of from approximately 15% to approximately 32% fiber. It is known that FRP articles have higher strengths at higher fiber loadings, but the prior art attempts to make sheet molding compounds of more than 50% fiber have resulted in nonuniform materials in which the percent of glass varied appreciably. These materials have the serious drawback that products made therefrom have an even greater decrease in fiber loading in the extruded areas where the fibers are most needed. A still further shortcoming is that sheet molding compounds can not be made of a thickness of more than approximately 1/4 inch; and delamination occurs when parts are made from multiple layers of the sheet molding compound. Another problem is that variations in the percent of glass leads to "short shots" i.e. parts made when the mold is incompletely filled. Glass weighs more than resin. A charge which weighs the correct amount but which has too high a glass content, does not have sufficient volume to fill the mold and a "short shot" thereby results.
An object of the present invention is the provision of a new and improved method of making FRP articles and/or materials wherein the materials can be deeply extruded without producing resin rich areas, and wherein the fibers move along with the matrix forming materials into the extruded areas.
Another object of the invention is the provision of a new and improved method of the above described type wherein parts of thicknesses commonly encountered in the metal casting and/or diecasting arts can be produced, and wherein the FRP material will flow into fillets and webs without depletion in fiber loading.
Another object of the invention is the provision of a new and improved method of the above described type wherein parts having a fiber loading of more than 50% can be easily produced.
Another object of the invention is the provision of a new and improved process for causing chopped glass fibers to lay down in a more planar and compact arrangement than has been produced heretofore.
Another object is the production of a press charging billet having a precise glass fiber to resin ratio in a highly automated manner.
A still further object of the invention is the provision of a new and improved process for producing and curing of the billets in maturation tubes to facilitate handling and reduce the storage space required for maturation.
Still further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates from the following description of several preferred embodiments that are described with reference to the accompanying drawings which form a part of this specification.